Photoemission study of onionlike carbons produced by annealing nanodiamonds

Photoelectron spectroscopy has been used to study the products resulting from high temperature phase transformation of nanodiamonds (ND). Depending on the temperature of annealing various particles with a diamond core covered by nanometer sized fullerene-like shells, and onionlike carbon (OLC) were formed. Analysis of the C1s photoemission lines of the intermediates of ND transformation, prepared at temperatures of 1420 and 1600 K and then exposed to atmosphere, reveals the presence of oxygen-containing groups and both sp(2) and sp(3) carbon. The sp(2) component for these samples has binding energies of 284.7 +/- 0.05 eV (for the sample prepared at 1420 K) and 284.50 +/- 0.05 eV (for the sample prepared at 1600 K). A difference of 1.3 +/- 0.1 eV in the binding energy of the sp(3) and sp(2) components was observed. The sp(2) component for OLC prepared at 1800, 1900, and 2140 K has a binding energy of 284.45 +/ -0.05 eV. The shift towards higher binding energies of the sp(2) component of the samples prepared at lower temperatures is explained by significant curvature of graphite layers formed in the initial stages of graphitization. The observed increase in density of states at the Fermi level for the samples prepared at 1600, 1800, and 1900 K is associated with an accumulation of different types of defects in the curved graphite layers during graphitization of diamond. The Lorentzian widths of C1s photoemission lines from OLC are large compared with those of HOPG. The possible reasons for this broadening are discussed.